Scientific researches indicate that human's normal sleep structural period can be divided into two phases which are non-rapid eye movement (NREM) term and rapid eye movement (REM) term. The NREM and the REM appear alternately, and one alternation is called one sleep period. Each night usually has 4-5 sleep periods, and each period has 90-110 minutes.
During the NREM stage, the human body is in a sound sleep and deep sleep state. The brain wave has a low frequency and high amplitude, and presents a slowly varying curve. In addition, the eyeballs will not move rapidly. At this time, the sleep has large inertia, and the person is not easy to be awaked. If waking up from the NREM stage, a person will feel dizzy and sleepy, act slowly and be in a bad mood.
During the REM stage, the human body is in a shallow sleep state. The brain wave has a high frequency and low amplitude, and presents a saw tooth wave curve with distinctive features. The eyeballs move rapidly between right and left, the heart rate increases, and the breath is fast and irregular, the body temperature goes up, and dreams appear. Because the sleep is relatively shallow, the sleep inertia is small, and thus the person is easy to be awaked. If waking up from the REM stage, a person will feel a clear mind, act agilely, and be full of spirit and energy.
Existing alarm clocks awake a human body forcibly no matter in which sleep stage the human body is, which is very bad for human health if it acts in such a way for a long time.